1. Field of the Invention
This invention relates to an image sensing device which is arranged to permit selection of either a field reading mode (hereinafter referred to as the field mode) or a frame reading mode (hereinafter referred to as the frame mode).
2. Description of the Related Art
There are electronic still cameras of the kind arranged to permit a selection between a field image signal and a frame image signal in forming an image signal within an image sensor before reading it out for recording. An example of the camera of this kind is disclosed in Japanese Laid-Open Patent Application No. SHO 58-48455.
FIG. 6 of the accompanying drawings shows the switching circuit of an electronic still camera of the above stated kind arranged to permit a selection between the field mode and the frame mode. The illustration includes an adder 60; signals 62.sub.1 and 62.sub.2 produced from odd-number field picture elements (hereinafter referred to as ODD picture elements) and even-number field picture elements (hereinafter referred to as EVEN picture elements) respectively; change-over switches S1 and S2 which are provided for a selection between the field mode and the frame mode; field mode selecting contacts A1 and B1; frame mode selecting contacts A2 and B2; a terminal A3 which is provided for reading a field image; and terminals B3 and B4 which are provided for reading the odd- and even-number field portions of the frame image signal, respectively. When the moving contacts of the field mode/frame mode change-over switches S1 and S2 are on the side of the frame mode selecting contacts A2 and B2, the information of the ODD picture elements and that of the EVEN picture elements are serially read out from the reading terminals B3 and B4 for every field. In case that the moving contacts of the change-over switches S1 and S2 are on the side of the field mode selecting contacts A1 and B1, the information of the ODD picture elements and that of the EVEN picture elements are added together by the adder 60 and read out from the reading terminal A3 as a field image.
FIG. 7(A) shows relations obtained between the quantity of incident light and the intensity of an output signals obtained in the field mode and in the frame mode. FIG. 7(B) shows a relation between the illuminance of a object and the intensity of the output signal obtained in the frame mode with the incident light quantity optically reduced to 1/2 by means of an ND filter or an exposure device. Referring to these drawings, reference symbols Vs1 and Vs2 denote saturation output voltages obtained in the frame mode and in the field mode. A symbol Hs denotes saturation light quantity on the image sensing plane. Symbols Ls1 and Ls2 denote saturation degrees of object's illuminance obtained in the frame mode and in the field mode respectively. As apparent from the characteristic curves of FIG. 7(A), the sensitivity obtained on the image sensing plane is higher by two times in the field mode than in the frame mode.
The electronic still camera is generally arranged to perform a process of suppressing the high luminance portion of an image signal for obtaining an effect called a knee effect (hereinafter referred to as a knee process) with regard to the problem relative to the dynamic ranges of the image sensor and signal processing system or the visual sensation characteristic of human. FIGS. 8(A), 8(B), 9(A) and 9(B) respectively show the result of the knee process applied to the characteristics shown in FIGS. 7(A) and 7(B). In these drawings, a reference symbol Vk denotes a knee point voltage which is the start voltage of the knee process; and a symbol Vmax denotes a maximum signal level determined by the dynamic range of the signal processing system. Further, FIGS. 8(A) and 9(A) show the results of the knee process carried out in such a way as to cause the saturation voltage obtained in the frame mode to become the above stated maximum signal level Vmax. FIGS. 8(B) and 9(B) show the result of the knee process carried out to cause the saturation voltage obtained in the field mode to become the voltage Vmax.
As apparent from these drawings, the output signal intensity obtained in the field mode with the knee process carried out to bring the saturation voltage of the frame mode to the voltage Vmax comes to exceed the dynamic range of the signal processing system. Whereas, in case that the knee process is carried out to bring the saturation voltage obtained in the field mode to the voltage Vmax, the output signal intensity does not reach the voltage Vmax. In the latter case, therefore, there takes place some contrast blunting effect.
Further, in the case of the conventional image sensing device arranged to obtain the sum of the electric charge values of two picture elements within the sensor, the problem of blooming has occurred when the sum comes to exceed the saturation potential of the V-CCD thereof.